There is a limit to the size of the battery pack in the Model 3, both on a weight basis and on a volumetric basis. As a result, I'm expecting the base battery pack to be around 50-55 kWh, and the bigger pack to be around 70 kWh.
Right now, Ludicrous mode allows a discharge c-rate around 5.6C. Assuming that the increase in specific energy expected with a battery chemistry change in the Model 3 doesn't negatively affect the discharge c-rate (not necessarily a safe assumption), then we're looking at 5.6 * 70 = 392 kW, or 525 hp. But the battery pack likely weighs in at 390 kg, making the total weight of the car around 4,200 pounds. A P90D+L and P100D+L are around 5,000 pounds, so that's a lot of weight shaved off.
I can definitely see under 3 seconds if these parameters hold. Maybe 2.8 or 2.9?'
Note that for c-rate discharge, I would use the entire pack's capacity, but for range calculation, one has to factor in the anti-bricking buffer of about 5%.
Note... thinking about it more, I'm thinking the upper end is more likely 65 kWh, or 488 hp, and getting about 2.9 or 3.